Nickel direct-plating

ABSTRACT

A method of depositing nickel on a surface of an object, the method including the steps of providing a source of direct current having a positive and a negative terminal; connecting the object to the negative terminal; connecting an anode to the positive terminal; and submerging the object and anode in a solution comprising nickel. The anode is positioned at a distance equal to or less than 2 mm from the surface of the object and when the source of direct current is switched on, nickel in the solution comprising nickel is deposited on the surface of the object.

The present invention relates to a method of depositing nickel or nickel alloys onto aluminium and its alloys and titanium and its alloys. In particular, the present invention relates to a method of depositing nickel onto aluminium and nickel onto titanium.

Aluminium is the second most widely used metal after iron. Aluminium is used in the manufacture of many household appliances, aerospace and automotive products and is lightweight, soft, durable, and ductile.

It is common to want to plate other metals onto the surface of an aluminium or titanium object for decoration or to enhance the physical properties of the aluminium or titanium. Nickel is often plated onto the surface of aluminium or titanium to improve the corrosion resistance of the aluminium or titanium. It may also improve the wear resistance of the aluminium or titanium and/or increase its luster.

Nickel and other metals are often plated onto the surface of aluminium or titanium by a process called electroplating. The electroplating process uses an electrolyte and an electric current. The electric current is applied between the aluminium or titanium, the substrate (cathode), and an electrode (anode). The process is a chemical redox reaction. The process deposits a layer of nickel or another metal onto the aluminium or titanium, the substrate.

It is known however that as soon as aluminium and/or titanium are exposed to an atmosphere including oxygen, a thin oxide layer forms on the outer surface of the aluminium or titanium. This thin oxide layer affects the adhesion between the aluminium or titanium and the nickel or other metal that is to be deposited onto the surface of the aluminium or titanium. In order to achieve effective adhesion, the oxide layer is removed and prevented from reforming. Before electroplating, the aluminium or titanium is therefore usually pre-treated and/or pre-plated with an intermediate metallic layer. The aluminium or titanium may be cleaned to remove contaminants including the oxide layer that may interfere with the flow of ions between the anode and the aluminium or titanium.

It is known to pre-plate the aluminium or titanium by immersing it in a solution of zinc or tin. It is also known to apply multiple layers of nickel or another metal to the exposed surface of the aluminium or titanium. The function of pre-plating and/or multiple layers of nickel is to improve the adhesion of the nickel or other metal to be applied to the aluminium or titanium. The adhesion may be molecular adhesion.

It is known to apply a thin coating of nickel to the surface of the aluminium or titanium, then heat the nickel-clad aluminium or titanium to form a suitable base upon which any subsequent metal can be plated with improved adhesion.

It is also known to first clean the surface of the aluminium or titanium with a solution of sodium or potassium hydroxide. The solution typically has a pH of ca. 12 or above. The solution may also comprise nickel or cobalt. A non-cyanide complexing agent is used to keep the nickel or cobalt in solution at a pH of ca. 12 or above.

In accordance with a first aspect of the present invention there is provided a method of depositing nickel on a surface of an object, the method including the steps of:

-   -   providing a source of direct current having a positive and a         negative terminal;     -   connecting the object to the negative terminal;     -   connecting an anode to the positive terminal; and     -   submerging the object and anode in a solution comprising nickel;     -   wherein the anode is positioned at a distance equal to or less         than 2 mm from the surface of the object and wherein when the         source of direct current is switched on, nickel in the solution         comprising nickel is deposited on the surface of the object.

The nickel may be directly deposited on the surface of the object. This may be referred to as direct plating.

The step of submerging the object and anode in a solution comprising nickel may be referred to as positioning.

The object may comprise one or more of aluminium, titanium, stainless steel and molybdenum. The surface of the object may be coated in an oxide layer.

Stainless steel is typically an alloy of iron, carbon and chromium. Metals such as nickel, molybdenum and titanium may be added to the alloy to enhance properties such as strength.

The method of depositing nickel on the surface of the object may include the step of removing the oxide layer before the nickel in the solution comprising nickel is deposited on the surface of the object.

It may be an advantage of the present invention that the oxide layer is removed from the surface of the object before the nickel in the solution comprising nickel is deposited on the surface of the object and that removing the oxide layer may be part of the step of depositing the nickel on the surface of the object.

A conventionally used separate method step to remove the oxide layer typically increases the overall cost of operating the method because of the increased time in production and/or the additional raw materials required.

This may be particularly suited to the object when it comprises titanium, the object coated in a titanium oxide layer.

The method of depositing nickel on the surface of the object may alternatively include the step of thickening the oxide layer before the nickel in the solution comprising nickel is deposited on the surface of the object.

It may be an advantage of the present invention that the oxide layer is thickened and/or the surface of the object becomes susceptible to the nickel in the solution comprising nickel being deposited on the surface of the object. Thickening of the oxide layer may be part of the step of depositing the nickel on the surface of the object.

This may be particularly suited to the object when it comprises aluminium, the object coated in an aluminium oxide layer.

The inventor of the present invention considers the relatively small atomic size of nickel ions compared to aluminium oxide may mean that the nickel ions are able to penetrate the atomic sized gaps between the aluminium oxide particles. The aluminium oxide particles may form a layer. The nickel ions may nucleate to form a mono-layer that may then grow out from the surface of the object comprising aluminium. This may increase the level or degree of adhesion between the surface of the object comprising aluminium, also referred to as the aluminium substrate, and the nickel ions.

The aluminium oxide layer may comprise microscopic pores. The microscopic pores may be nano-sized pores. According to one embodiment of the present invention the nickel is deposited onto the surface of the object comprising aluminium through microscopic and/or nano-sized pores in the aluminium oxide layer.

The shape of the object and anode may be complementary. The shape of the object and anode may be such that the inner surface area of the anode is substantially the same as the outer surface area of object. In an alternative embodiment the outer surface of the anode is substantially the same as the inner surface of the object.

The surface area of the anode and the surface area of the object in contact with the solution comprising nickel may be substantially the same. The surface area of the anode may be from 50 to 100% of the surface area of the object, typically from 70 to 100% and normally from 90 to 100%.

The method may further include the step of anodising an outer surface of the object.

The method of depositing nickel on the surface of the object may use a current density of from 10 A/dm² to 70 A/dm², typically from 30 A/dm² to 50 A/dm². This very high current density may help to prepare the object for accepting the nickel.

The current density at which the nickel is satisfactorily deposited on the surface of the object is typically affected by the rate at which the solution comprising nickel flows over the surface of the object.

The high electric potential may help the deposition of nickel on the surface of the object. Increasing the electric potential may increase the rate at which nickel is satisfactorily deposited on the surface of the object.

The object connected to the negative terminal may be referred to as the cathode.

The solution comprising nickel may be referred to as an electrolyte. The nickel in the solution comprising nickel is typically ionic, that is charged. The nickel in the solution is typically in form of Ni⁺, Ni²⁺, Ni³⁺, or Ni⁴⁺. The nickel in the solution is normally in the form of Ni²⁺, also referred to as Ni(II).

When the source of direct current is switched on, the nickel in the solution comprising nickel is deposited on the surface of the object. When the source of direct current is switched on the nickel in the solution comprising nickel is typically oxidised at the anode from Ni with a zero valency to positive valency of one of Ni⁺, Ni²⁺, Ni³⁺, or Ni⁴⁺ having a valency of +1, +2, +3 or +4 respectively. Nickel with a positive valency is attracted to the object that is connected to the negative terminal.

The electrolyte may comprise one or more of nickel sulphate (NiSO₄.6H₂O), nickel chloride (NiCl₂.6H₂O), boric acid (B(OH)₃), Watts-type, nickel sulphamate (Ni(NH₂SO₃)₂.4H₂O), nickel fluoborate (Ni(BF₄)₂), all-chloride, sulphate-chloride, all-sulphate, hard nickel and black nickel.

The concentration of the one or more of nickel sulphate (NiSO₄.6H₂O), nickel chloride (NiCl₂.6H₂O), boric acid (B(OH)₃), Watts-type, nickel sulphamate (Ni(NH₂SO₃)₂.4H₂O), nickel fluoborate (Ni(BF₄)₂), all-chloride, sulphate-chloride, all-sulphate, hard nickel and black nickel in the electrolyte may be from 0 to 500 g/L.

The nickel deposited on the surface of the object may be a nickel containing alloy. The nickel containing alloy may comprise one or more of cobalt and tungsten.

The anode may comprise a metal, the metal may be an inert metal. In an alternative embodiment the anode may comprise nickel.

The method may further include the step of heating the solution comprising nickel. The solution comprising nickel may be heated to from 30 to 80° C., typically from 40 to 75° C. As the temperature of the solution comprising nickel is increased, the rate the nickel in the solution comprising nickel can be deposited on the surface of the object also increases. The solution comprising nickel may be heated using a hot plate. The temperature of the hot plate may be thermostatically controlled.

The distance equal to or less than 2 mm from the surface of the object and the anode may be referred to as a gap. The method may further include the step of pumping the solution comprising nickel through the gap. The solution comprising nickel may flow through the gap. The solution comprising nickel may be pumped through the gap at a speed of from 0.5 to 10 m/s, typically from 1 to 5 m/s.

The solution comprising nickel may be continuously pumped through the gap, that is whilst the nickel is being deposited on the surface of the object according to the method of the present invention, there is no interruption in the flow of solution comprising nickel through the gap.

The speed of the flow of the solution comprising nickel through the gap may be such that the method of depositing nickel on the surface of the object may be referred to as one or more of high-speed, ultra-high-speed and high-speed solution movement.

It is an advantage of the present invention that the method of depositing nickel may not burn the surface of the object. Burning the surface of the object will typically cause the surface and/or the nickel deposited on the surface to blacken in colour.

The speed of the flow of the solution comprising nickel through the gap, and therefore the speed of the flow of the solution comprising nickel that can contact the surfaces of the object, may help to push the nickel, in the solution comprising nickel, through the microscopic pores in the oxide layer and into contact with the surface of the object.

It is an advantage of the present invention that the flow of the solution comprising nickel is such that the solution can carry more electrical current than other comparable systems used for electrodeposition. The solution may be able to carry ten times the electrical current compared to other systems.

The speed of the flow of solution through the gap and the current density may help the method of depositing nickel according to the present invention not to burn the surface of the object.

The object, the anode and the solution comprising nickel may be held in a container. The container may be referred to as a cell. The container typically holds the solution comprising nickel.

The negative terminal and/or the positive terminal may be connected to a rectifier. The rectifier may convert a supply of alternating current (AC) to provide the source of direct current.

The anode may be inside or outside the object. When the anode is outside the object, the nickel in the solution comprising nickel may be deposited on the outer surface of the object. When the anode is inside the object, the nickel in the solution comprising nickel may be deposited on the inner surface of the object.

The speed of the flow of the solution comprising nickel through the gap may be different depending on whether the method is being used to deposit nickel on the inside or outside surface of the object.

The method according to the first aspect of the present invention may be particularly suited to depositing nickel on the surface of a tubular object. The tubular or tubular-shaped object may be one or more of a rod, tube and pipe. The tubular object may have a uniform internal diameter. The surface may be curved. The method according to the first aspect of the present invention may be particularly suited to depositing nickel on a curved surface.

The object comprising aluminium may be stainless steel.

The flow of the solution comprising nickel over the surface of the object may be turbulent. Turbulent flow may be non-linear.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional diagram of one embodiment of the present invention; and

FIG. 2 is a schematic cross-sectional diagram of another embodiment of the present invention.

FIG. 1 shows a schematic cross-sectional diagram of the apparatus 10 used to deposit nickel 12 on surfaces 14 of an object 16.

A source of direct current 20 has a positive terminal 22 and a negative terminal 24. The object 16 is connected to the negative terminal 24 by a wire 26. An anode 18 is connected to the positive terminal 22 by a wire 28. The object 16 and anode 18 are housed in a cell 80.

In use the object 16 and anode 18 are submerged in a nickel solution 30.

The anode 18 is positioned at a distance equal to or less than 2 mm from the surfaces 14 of the object 16. When the source of direct current 20 is switched on, the nickel 12 from the nickel solution 30 is deposited on the surfaces 14 of the object 16.

When the object comprises titanium, the surfaces 14 of the titanium object 16 are coated in a titanium oxide layer (not shown). The titanium oxide layer (not shown) is removed from the surfaces 14 of the titanium object 16 by the nickel solution 30 and before the nickel 12 is deposited on the surfaces 14. Removal of the titanium oxide layer (not shown) is part of the step of depositing the nickel 12 from the nickel solution 30 on the surfaces 14.

When the object comprises aluminium, the surfaces 14 of the aluminium object 16 are coated in an aluminium oxide layer (not shown). The aluminium oxide layer (not shown) may comprise nano-sized pores. When the object comprises aluminium, the nickel 12 is deposited onto the surfaces 14 of the aluminium object 16 through the nano-sized pores in an aluminium oxide layer (not shown).

The direct current 20 has a current of 0.5 amps and a current density of 70 A/dm². The current density depends on the surface area of the cathode. The direct current is provided by a power generator (not shown) via a rectifier (not shown). The rectifier converts a supply of alternating current (AC) to provide the source of direct current 20.

In use, the nickel solution 30 flows over the surfaces 14 of the object 16. The nickel solution 30 may be referred to as an electrolyte. The nickel 12 in the nickel solution 30 is ionic, and is in form of Ni²⁺.

When the source of direct current 20 is switched on, the nickel 12 in the nickel solution 30 is deposited on the surfaces 14 of the object 16. When the source of direct current 20 is switched on the nickel 12 in nickel solution 30 is oxidised at the anode 18 from Ni with a zero valency to positive valency of Ni²⁺. The Ni²⁺ with a positive valency is attracted to the object 16 that is connected to the negative terminal 24.

In use the nickel solution 30 is heated to a desired temperature of 60° C., and typically from 50 to 80° C. using a thermostatically controlled hot plate 50. The nickel solution 30 is held in a bath 52 on the hot plate 50.

The electrolyte or nickel solution 30 is a Watt's-type nickel electrolyte consisting of nickel sulphate (NiSO₄.6H₂O), nickel chloride (NiCl₂) and boric acid.

The Watt's-type nickel electrolyte comprises:

-   -   Nickel sulphate, NiSO₄6H₂O: from 32 to 40 oz/gal (from 240 to         300 g/l);     -   Nickel chloride, NiCl₂6H₂O: from 4 to 12 oz/gal (from 30 to 90         g/l); and     -   Boric acid, H₃BO₃: from 4 to 6 oz/gal (from 30 to 45 g/l).

In use, the nickel solution 30 is regularly monitored and the concentration of the nickel in solution maintained to ensure optimum conditions for the deposition of nickel 12 on the surfaces 14 of the object 16. A control unit 60 is used to monitor and control the concentration of the nickel sulphate in the nickel solution 30. The control unit 60 is in fluid communication with the nickel solution 30 in the bath 52 via a conduit 54.

The distance between the surfaces 14 of the object 16 and the anode 18 is equal to or less than 2 mm and is referred to as a gap 32. In use the nickel solution 30 is pumped through the gap 32 using a pump 70. In use the nickel solution 30 flows through the gap 32 at a speed or flow rate of from 3 to 5 m/s.

The pump 70 is used to pump the nickel solution 30 from the bath 52, along conduits 56 a and 56 b and into the cell 80. The nickel solution 30 flows between the anode 18 and the surfaces 14 of the object 16. The nickel solution then returns to the bath 52 from the cell 80 via a conduit 58.

The pump 70 controls the flow rate of nickel solution 30 through the cell 80. Other devices (not shown) may also be used to control the flow rate of nickel solution 30 through the cell 80. The flow path of the nickel solution 30 may be referred to as a hydraulic circuit. The arrows in FIG. 1 show the direction of the flow of the nickel solution 30. The apparatus 10 can be described as a closed-loop system because the nickel solution 30 is recycled through the cell 80.

In use the nickel solution is circulated through the cell 80 until the nickel 12 on the surfaces 14 of the object 16 are the desired thickness.

The apparatus 10 may include a filter (not shown). The filter may be on the end of the conduit 56 a that is in the bath 52. The filter may be used to ensure clean nickel solution 30 enters the pump 70 and cell 80. It removes any foreign particles suspended in the nickel solution 30 from being co-deposited with the nickel 12.

A flow meter (not shown) may be used to measure the flow rate of nickel solution 30 through the cell 80.

The features of the apparatus 110 shown in FIG. 2 are mostly the same features of the apparatus 10 shown in FIG. 1. Comparable features of the apparatus 10 and 110 have been labelled using reference numerals, differentiated by a factor of 100. Feature 16 in FIG. 1 is for example feature 116 in FIG. 2.

FIG. 1 shows the anode 18 outside the object 16. When the anode 18 is outside the object 16, the nickel 12 from the nickel solution 30 is deposited on the outer surfaces 14 of the object 16.

FIG. 2 shows the anode 118 inside the object 116. When the anode 118 is inside the object 116, the nickel 112 from the nickel solution 130 is deposited on the inner surfaces 115 of the object 116.

The speed of the flow of the nickel solution 130 through the gap 132 in FIG. 2 is different to the speed of the flow of the nickel solution 30 through the gap 32 in FIG. 1.

The object 16, 116 is a pipe having a tubular cross-section and uniform internal and external diameter. The inner 115 and outer 14 surfaces of the object 16, 116 are curved.

As shown in FIG. 1, the shape of the inner surface of the anode 18 and outer surface 14 of the object 16 are complementary, that is the area of the two surfaces is substantially the same. As shown in FIG. 2, the shape of the inner surface 115 of the aluminium object 116 and outer surface of the anode 118 are complementary, that is the area of the two surfaces is substantially the same.

The surface areas of the anode 18, 118 and object 16, 116 in contact with the nickel solution 30, 130 respectively are substantially the same.

Modifications and improvements can be incorporated without departing from the scope of the invention. 

1. A method of depositing nickel on a surface of an object, the method including: providing a source of direct current having a positive and a negative terminal; connecting the object to the negative terminal; connecting an anode to the positive terminal; and submerging the object and anode in a solution comprising the nickel; wherein the anode is positioned at a distance equal to or less than 2 mm from the surface of the object, and wherein when the source of direct current is switched on, the nickel in the solution comprising the nickel is deposited on the surface of the object.
 2. A method according to claim 1, wherein the nickel is directly deposited on the surface of the object.
 3. A method according to claim 1, wherein the surface of the object is coated in an oxide layer, the method including removing the oxide layer before the nickel in the solution comprising the nickel is deposited on the surface of the object.
 4. A method according to claim 1, wherein the surface of the object is coated in an oxide layer, the method including thickening the oxide layer before the nickel in the solution comprising the nickel is deposited on the surface of the object.
 5. A method according to claim 1, wherein the object comprises at least one of aluminium, titanium, stainless steel and molybdenum.
 6. A method according to claim 1, wherein a shape of the object and a shape of the anode are complementary, and an inner surface area of the anode is substantially equal to an outer surface area of the object.
 7. A method according to claim 1, wherein a shape of the object and a shape of the anode are complementary, and an outer surface area of the anode is substantially equal to an inner surface area of the object.
 8. A method according to claim 1, wherein the method further includes anodising an outer surface of the object.
 9. A method according to claim 1, wherein the source of direct current has a current density of from 10 A/dm² to 70 A/dm².
 10. A method according to claim 1, wherein the nickel in the solution is in a form comprising at least one of Ni⁺, Ni²⁺, Ni³⁺ and Ni⁴⁺.
 11. A method according to claim 1, where the solution comprising the nickel is an electrolyte, the electrolyte comprising at least one of nickel sulphate (NiSO₄.6H₂O), nickel chloride (NiCl₂.6H₂O), boric acid (B(OH)₃), Watts-type, nickel sulphamate (Ni(NH₂SO₃)₂.4H₂O), nickel fluoborate (Ni(BF₄)₂), all-chloride, sulphate-chloride, all-sulphate, hard nickel and black nickel.
 12. A method according to claim 11, wherein a concentration of the at least one of nickel sulphate (NiSO₄.6H₂O), nickel chloride (NiCl₂.6H₂O), boric acid (B(OH)₃), Watts-type, nickel sulphamate (Ni(NH₂SO₃)₂.4H₂O), nickel fluoborate (Ni(BF₄)₂), all-chloride, sulphate-chloride, all-sulphate, hard nickel and black nickel in the electrolyte is from 0 to 500 g/L.
 13. A method according to claim 1, wherein the method further includes heating the solution comprising the nickel to from 30 to 80° C.
 14. A method according to claim 1, wherein the distance equal to or less than 2 mm from the surface of the object and the anode is a gap, and the method further includes pumping the solution comprising the nickel through the gap at a speed of from 0.5 to 10 m/s.
 15. A method according to claim 1, wherein a flow of the solution comprising the nickel over the surface of the object is turbulent. 